1. Field of the Invention
The present invention relates to a data recording/reproducing apparatus which performs data recording/reproducing operations on a sector by sector basis using a disk-shaped information recording medium.
2. Description of the Prior Art
On the optical disk, optically detectable guide tracks are provided concentrically or spirally, and data are recorded by irradiating a laser beam, which is contracted to a diameter of less than 1 .mu.m, onto the guide tracks. Also, each track formed on the disk is divided into plural sectors. Data are recorded on a sector by sector basis.
An explanation of the sector format follows with reference to the drawings. FIG. 4 is a schematic diagram of a sector format used for a conventional data recording/reproducing apparatus. In FIG. 4, incorporated at each sector are an ID (identification) field 1 having recorded therein address information which includes a track address and a sector address, and a data field 2 having sector data recorded therein. An initial region of the data field 2 has a VFO Field 3 formed therein in which a cyclic signal is recorded for synchronization of a PLL (Phase Locked Loop), and a subsequent region of the data field 2 is divided into plural frames (in the FIG. 4, there are m frames numbering from Frame 1 to Frame m). The plural frames of the data sector include m sets of frame data (from F1 to Fm), respectively. Further, in an initial region of Frame 1 just prior to the first frame data F1, a data mark DM is added which is a synchronous signal for identifying the beginning of the frame data. Thus, recorded in the Frame 1 is a data mark field 4 and a frame data field 6. In an initial region of the subsequent Frames 2 to m just prior to the frame data Fn (n=2 to m), a frame mark FM is added which is a resynchronization signal for identifying the beginning of a frame. Thus, recorded in each Frame n (n=2 to m) is a frame mark field 5 and a frame data field 6.
The reasons for recording the sector data as a plurality distinct frame data is to protect against uncorrectable data errors resulting from a bit slip phenomenon. Namely, burst dropout may occur during signal reproduction due to defects, dust, flaws, etc., contained, for example, in the substrate, recording film, or protective layer composing an optical disk. When a burst dropout occurs, data is generated having a slip due to an abnormal increase or decrease in the number of read clocks, and the bit slip phenomena, in which all the data thereafter contains errors, may result. However, if the sector data are divided and recorded in plural distinct frames, even if such a phenomena occurs in one frame, the data error will be limited to the one frame and a shift of the read clock is compensated by locating the next frame mark, thus making it possible to carry out normal data reproduction in the next and subsequent frames.
When reproducing data from a target sector in an information recording medium having the above-described sector format, the data recording and reproducing apparatus carries out normal data reproduction by detecting the data mark from a reproduced signal and then triggering the demodulation. Also, when the data mark cannot be detected due to a defect, the data recording/reproducing apparatus will instead trigger the demodulation upon detecting the frame mark recorded in the 2nd frame. When scanning an unrecorded sector in which data have not been recorded at all in the data field, reproduction is not triggered because neither the data frame nor a frame mark will be detected, and the data recording/reproducing apparatus confirms that the target sector is an unrecorded sector by, for example, detecting an envelop signal which shows the existence or non-existence of the reproduced signal.
In order to implement high density recording, guide tracks having a narrow track pitch are formed on the recording medium disk. Stress may occur in the formation of the laser beam due to dislocation of a focusing control and tilting of the disk relative to the optical head.
The intensity of a laser beam on an optical disk generally exhibits a Gaussian distribution. Therefore, the intensity of a crosstalk signal, which is a reproduced signal from an adjacent track, increases as the track pitch is narrowed and the adjacent track is irradiated due to dislocation of the laser beam. In the case where an unrecorded target sector has adjacent thereto a recorded track, the data recording/reproducing apparatus can erroneously trigger demodulation upon identifying a data mark contained in a crosstalk signal from the adjacent track and thus behave as if the data recorded in the adjacent track is instead recorded in the target sector. If such a condition occurs, the reliability of data reproduced from the target sector is substantially lowered.